Such a method is known under the designation of "Super-COSY" and is described inter alia in the book of Chandrakumar/Subramanian "Modern Techniques in High Resolution FT NMR", Springer-Verlag, pages 177-181. According to FIG. 4.41 of this publication, the first and third pulses are 90.degree. pulses, which, like the two 180.degree. pulses, are coherent in phase; therefore, during these pulses the carrier oscillation of the high-frequency magnetic field has a defined phase position. The nuclear magnetic resonance signal, which develops after an instant which has the same time distance from the second 180.degree. pulse as the latter has from the third pulse, is transposed to the low-frequency range, is digitized and is stored. During the further repetitions of this sequence, the distance between the first high-frequency pulse and the first 180.degree. pulse is varied.
The nuclear magnetic resonance signals then obtained constitute a two-dimensional set of data, whose data depend upon the time distance of the relevant sample value from the second 180.degree. pulse and on the time distance between the two first high-frequency pulses. If through each of these variables--each time diminished by the distance of the third pulse from one of the two 180.degree. pulses--a Fourier transformation is carried out, a two-dimensional spectrum is obtained, in which the frequency lines of two molecule groups coupled to each other appear on a diagonal line (diagonal peaks) and appear symmetrically to this diagonal (cross-peaks).
By means of this method, various of the amino acids also present in the human body, such as, for example, the glutamates, glutamin, taurins and the inosites, can be demonstrated by small concentrations--but only in vitro. In examinations in vivo at the human body, additional difficulties are met, because as a rule only the spectrum of a subvolume of the examination area is of importance (localized spectroscopy) and because moreover at the examination area protons bound to water are present, whose concentration is several powers of tens higher than the concentration of the protons in the relevant amino acids.
From the magazine "Magnetic Resonance in Medicine", 6, pp. 334-343 (1988), a method for the localized correlation spectroscopy (COSY) is known, in which in a sequence after two 90.degree. pulses, whose relative distance is varied, three 180.degree. pulses are produced at a fixed distance from the second pulse, which are accompanied by magnetic gradient fields, whose gradients extend at right angles to each other. The echo signal is then processed. This "adhesion" of the three layer-selective 180.degree. pulses to the actual "COSY" sequence has the disadvantage that the time from the first excitation pulse to the sampling of the nuclear magnetic resonance signal is considerably lengthened so that the amplitude thereof is considerably reduced inter alia due to relaxation effects. More particularly, the method cannot be used for proton spectroscopy in vivo because the water suppression is not sufficient.